I think I'm responding to Nick Bostrom, following some discussion
by Jason Thompson & Hal Finney, but I'm not sure given the
forwarding this went through.

>But finding a second source of intelligent life, in our relative vicinity,
>would dramatically boost the support for those theories which say that
>intelligent life is common. (My PhD thesis at
>http://www.anthropic-principle.com/phd has more on this and many other
>related topics.)

I think you may need to refine the term "intelligent life" a bit. A pretty
good case can be made that dolphins, whales, and even perhaps other
primates are as "intelligent" as we are. What they lack is a system
for recording and transmitting knowledge across generations. I think the
human ability to do this is what makes us interesting. So, you want
to really focus on the development of intelligent "technological" life.
Of interest, might also be species, that are much less intelligent, but
do develop the ability to archive information, then retreive and manipulate
it. Hive minds of insects that developed external information storage
technologies might be very interesting. I believe there was
an example of this in Permutation City.

>For example, we can compare
>the time it actually took for intelligent life to evolve and compare that
>to the time that was available (an upper bound on which is given by the
>time when the sun engulfs the earth in about five billion years).

Hsssss... The sun engulfing the planet is a very poor limit unless
you are taking a very anthropic view of what "life" must look like.
The habitable water zones shrink as the stars get smaller (K & M
class), but they are much more numerous and live much longer. Free-formed
or wandering planets (flung out of solar systems by gravitational sling-shots)
could still evolve life based on energy from gravitational contraction
or radioactive decay if they formed in regions significantly enriched
by SN explosions with radioactive elements. While the matter density
of gas clouds is low, we know they are seedded with the building
blocks of life, have energy sources in nearby stars and have very
large (>> solar) masses (which may offset their low density).

Going back as far as Shklovsky and Sagan, there are concepts
of civilizations advanced only slightly beyond ours, to reengineer
their star to extend its longevity. There are also concepts for moving
the planet. So, any limits on the maximum longevity of technological
civilizations are very questionable.

So, using the model that "intelligent life" must evolve (and continue
to live) on water based planets around G class stars with 10 billion
year longevities is *not* a good approach to the discussion, IMO.
I recognize that because it represents the only known "existing" example,
it makes some "limited" sense.

>We can also look at our evolutionary record and try to identify plausible
>candidates for critical steps. For example, the step from prokaryotic to
>eukaryotic life seems to have been very difficult.

There were a number of messy problems that had to get solved in this
transition. There are two that seem the most important. The first
was the "active transport" of molecules to various locations in the cell
(the cellular postal system), which required the development of both the
transport motors and an addressing and delivery system. This was
a consequence of needing to make the cells larger to accomodate larger
genomes (genome packing may have also had to be developed). The
second was the isolation of the genome within the nucleus and the
complex machinery to handle duplicating & separating the chromosomes,
breaking down and reassembling the nuclei, etc. It isn't clear
exactly why this was done. The best argument I can make is the
protection of the DNA from free radicals or xenotoxins in the
cytoplasm, but this seems like a weak reason to me.

*However*, there is no reason to believe that "intelligent technological
life" requires eukaryotic architectures. There may be other equally
good solutions to the problems that eukaryotic cells were trying
to solve that could have occured elsewhere but may have taken
less evolutionary time. For example, there are bacteria, such
as Anabena, that do have sophisticated abilities to form cellular
aggregates with a division of labor between the cell types.
Is difficult to say how many different solutions might have
evolved here and died out due to changing climates, impacts
from space, etc. It still remains a mystery what happened
to the range of species produced in the precambrian explosion.
Who is to say what experimental approaches to even simpler
forms of life were developed prior to that that we have no
record of? On other planets, these could have taken faster
(or slower) approaches to the development of complex multi-cellular
forms.

>..., except that all the evidence we
>have is perfectly consistent with the hypothesis that it is extremely
>difficult for intelligent life to evolve. This is also consistent with (and
>in my opinion is the most probable explanation of) the absence of any signs
>of extraterrestrial intelligence (the Fermi paradox).

There is also a reasonable amount of evidence (IMO) to suggest that
there is no Fermi Paradox. We are almost completely lacking in the
ability to detect "life" at a distance, especially life at the limits
of physical laws. Minsky pointed out ~28 years ago that any "life"
that radiates energy at anything slightly above the background temperature
of the universe is being inherently wasteful. He is correct, and our
abilities to detect fine-scale variations in radiation at those temperatures
is very poor. So the only "question" that Fermi really raises is at what
point do intelligent civlizations choose to stop growing? I believe
that transition is one that every civilization must face as it
decodes its genetic makeup and develops the engineering to
create individuals that live indefinately. There comes a point
in time where "intelligence" (and philosophy?) must resolve
the points made by Malthus so long ago. Just as you can
envision a buckytube bearing being a "perfect" bearing, there
should be "optimal" architectures for life & civilizations
(depending on what they have consciously chosen to be and do).

By definition, expanding colonization of the galaxy (or the
universe!) cannot be one of those optima, because sooner or later
it must come to an end. Panspermia doesn't solve the problem of
"how does life originate", it just moves it someplace else.
The "Colonization Mentality" (which leads to the Fermi Paradox)
doesn't solve the problem of, "How do civilizations achieve a
balanced long term equilibria with their environment?"
(it simply postpones the inevitability of that requirement).
I don't think there is a "Fermi Paradox" to really advanced
civilizations as they would have refined their civilization
to a point where it is in balance with its environment.
There are a lot more PhD thesis that need to be written about
the possible end-points for technological civilizations and how
we might recognize them.